The present invention relates to an arrangement and a method for a brake cruise control, system of a vehicle. The invention further relates to a computer program adapted to perform such a method in a brake cruise control system when said program is run on a computer, and a computer program product comprising such program code means stored on a computer readable medium. The arrangement is suitable for heavy vehicles of different kinds having an auxiliary brake.
Modern vehicles are often provided with a cruise control system in order to improve the comfort of a driver. Some systems are also provided with a brake cruise control function that will also brake the vehicle if a set brake cruise speed is exceeded. The cruise control system may further be adapted to maintain a set cruise speed by using only the accelerator, or by also using the brakes of the vehicle. This is especially common in heavy vehicles which are equipped with auxiliary brakes, such as engine brakes and retarders. A further function of the cruise control system of the vehicle may be an economy drive setting, in which the regulating parameters of the cruise control system are ruled by economy.
The cruise control system has a regulating range in which the actual speed value is, maintained. For a system using only the accelerator to maintain the speed, an excessive speed of a few percent may be allowed. Typically, the speed may in this case vary between e.g. 69 to 71 km/h when the speed is set to 70 km/h. It may be possible to set the maximal allowed excessive speed value for the vehicle. In this way, the system can allow the vehicle to travel at an excessive speed at certain occasions, e.g. when travelling on a slight descent.
When the brake system is also used to maintain the speed of the vehicle, referred to as a brake cruise control system, a specified excessive speed can also be allowed for a vehicle rolling down a fairly steep downhill slope. An allowed excessive speed may help to preserve energy by using the momentum of the vehicle. The limit for the allowed excessive speed must of course be set to a value that does not endanger other vehicles or that exceeds speed limits. When an excessive speed limit is set, this value will be used as the set brake cruise speed. The driver sets the brake cruise speed to a value that he estimates that the auxiliary brakes of the vehicle can, maintain during a descent. If the auxiliary brakes cannot hold this set brake cruise speed, the driver must interact and brake the vehicle manually.
It is important that the set brake cruise speed is not exceeded when the vehicle travels downhill. The system compares the actual vehicle speed with the set cruise control speed, and applies the auxiliary brake when the actual vehicle speed equals the set brake cruise speed. Depending on the type or types of auxiliary brake/s available on the vehicle, the inclination of the downhill and the weight of the vehicle, the available brake power from the auxiliary brake may or may not be sufficient to hold the vehicle speed at the set brake cruise speed. For a heavy vehicle and/or a steep downhill, the vehicle speed may continue to rise even when the auxiliary brake is delivering full brake power. It is thus possible that the speed of the vehicle reaches a speed that is higher than the set brake cruise speed when the vehicle travels downhill. In some cases, depending e.g. on the inclination of the slope, the weight of the vehicle and the type of auxiliary brake, the reached speed may be higher than allowed. In such a case, the auxiliary brake may not be powerful enough to lower the speed back to the set brake cruise speed. Such a situation is not comfortable for the driver. Normally, the driver will apply the service brakes of the vehicle manually in order to decrease the speed.
Some drivers may apply the service brake forcefully for a short interval, such that the speed is reduced significantly, to e.g. a speed lower than the set brake cruise speed or even lower, and then release the service brake. By lowering the speed, the available brake power of the auxiliary brake may be enough to keep the speed at the set brake cruise speed. Some drivers may however apply the service brake constantly with a small amount, such that the speed of the vehicle is kept at the brake cruise speed. By applying the service brake constantly, the service brake will heat up which will lead to a degradation of the available brake power from the service brakes. Eventually, the service brake will be overheated and will lose all brake power. This situation should be avoided.
One way to avoid such a situation for the driver is to reduce the set brake cruise speed in order to avoid such situations in the future. But with a lower set brake cruise speed, the speed gap between the set cruise control speed and the brake cruise speed is reduced, which in turn increases fuel consumption and reduces the average speed of the vehicle.
It is known to monitor the road conditions in advance by using e.g. a GPS system in connection with a road map, such that the conditions ahead are known. In such a system, it is possible to decide in advance when the auxiliary brake is to be applied, if the speed of the vehicle is to be reduced before reaching the downhill or if the set brake cruise speed is to be modified, in order to lower the initial speed of the vehicle. However, most vehicles are not equipped with such a system.
US 2012/271524 describes a method and cruise control system for controlling a vehicle cruise control, where necessary downshifts can be avoided at certain vehicle driving situations. Based on the current vehicle condition and known road topography of the travelling road, a downshift can be predicted at a coming vehicle position in a coming uphill slope. By adjusting some vehicle parameters, the downshift can be postponed or avoided, for example a downshift from a direct gear which saves fuel.
WO 2007/139489 describes a system for controlling braking of a motor vehicle during downhill driving. When the auxiliary braking device is applied manually, the system calculates whether the auxiliary braking device is able to alone perform said braking action, based on detected vehicle parameters. If not, the system activates the service frictional braking device assist said auxiliary braking device.
US 2006/279137 describes a brake system in which an auxiliary brake is used as a primary brake and where a service brake may be used as a complement, when the brake power of the auxiliary brake is not enough. The system calculates how the vehicle will be driven in advance, such that suitable parameters of the vehicle can be selected.
US 2006/100768 describes a system in which the service brakes of a vehicle can be used as a complement to an auxiliary brake when a predefined setpoint speed is exceeded.
US 2006/113833 describes a system in which a service brake can be used as a complement brake to an auxiliary brake when the braking power of the auxiliary brake is reduced due to a decreased speed of the vehicle.
US 2012/283928 describes a vehicle control system in which the set brake cruise speed is adjusted by a control unit in dependency of different vehicle parameters, in order to take account of the delay in activation of an auxiliary brake and/or for comfort reasons.
These systems may work well in some situations, but there is still room for an improved cruise control system that can improve fuel efficiency and driver comfort.
It is desirable to provide an improved cruise control arrangement for a vehicle, with an improved activation of an auxiliary brake. It is also desirable to provide an improved method for activating an auxiliary brake in a cruise control system of a vehicle.
In a cruise control arrangement for a vehicle, where the cruise control arrangement is provided with a cruise control brake function and where the cruise control arrangement is provided with a set cruise speed value and a set brake cruise speed value, and where the cruise control brake function is active when the vehicle travels downhill, the problem is solved in that the cruise control arrangement is adapted to activate at least one auxiliary brake when the vehicle speed reaches the set brake cruise speed, to apply a service brake of the vehicle in order to reduce the speed of the vehicle to a predefined first speed if the vehicle speed exceeds the set brake cruise speed when the at least one auxiliary brake is delivering full brake power, where the predefined first speed is lower than the set brake cruise speed.
By this first embodiment of the cruise control arrangement, the arrangement will avoid undesired acceleration of the vehicle during downhill travel when the auxiliary brake cannot produce sufficient brake power. By the inventive arrangement, a downshift of the gearbox can be delayed, which can decrease the wear of the service brakes. When the vehicle speed reaches the set brake cruise speed, the auxiliary brake is activated. An auxiliary brake has a response time which is measured from the activation of the auxiliary brake until the auxiliary brake is fully applied and delivers the requested brake power. The response time is dependent on the type of auxiliary brake.
The speed of the vehicle is monitored, and if the speed increases when the auxiliary brake delivers full brake power, i.e. after the response time of the auxiliary brake, the service brake of the vehicle is activated in order to reduce the speed of the vehicle. The speed of the vehicle is reduced to a predefined first speed which is lower than the set brake cruise speed. The auxiliary brake is still delivering full brake power and the brake power of the auxiliary brake is not reduced. The service brake is released at the predefined first speed and the speed will again increase to the set brake cruise speed where the service brakes are once again applied. In this way, the speed of the vehicle can be held in a speed interval close to the set brake cruise speed without having to perform a downshift.
Since the brake power of the auxiliary brake, which is delivering full brake power, is not enough for holding the vehicle speed at the set brake cruise speed, the speed will slowly increase until the speed exceeds the set brake cruise speed. When the vehicle speed exceeds the set brake cruise speed, the service brake is applied in order to once again reduce the speed to the predefined first speed. When the vehicle speed equals the predefined first speed, the service brake is released.
If the speed of the vehicle increases again such that the vehicle speed exceeds the set brake cruise speed, the operation is repeated. It may also be that the inclination of the downhill slope has decreased, such that the brake power of the auxiliary brake is sufficient to hold the vehicle speed at the set brake cruise speed. In such a case the service brake will not be applied. If the speed increases again, the operation with the service brake is repeated.
In an advantageous development of the inventive arrangement, the operation is repeated until a predetermined condition is reached. The predetermined condition may e.g. be dependent on a predetermined number of repetitions, on the time interval between the application of the service brake, on the acceleration of the vehicle, on the remaining distance of the downhill or on the amount that the service brake has been used.
In an advantageous development of the inventive arrangement, downshift of the gearbox is performed when the predetermined condition is reached. As known in the art, with a downshift, the available brake power of an auxiliary brake arranged upstream of the gearbox will increase such that the brake power may be sufficient to hold the vehicle at a lower speed.
In a method for controlling braking of a vehicle having a brake cruise control function when the vehicle is travelling downhill and when the brake cruise control function is active, where the cruise control brake function is provided with a set cruise speed and a set brake cruise speed, the steps of, activating at least one auxiliary brake when the vehicle speed reaches the set brake cruise speed, of determining if the at least one auxiliary brake is delivering full brake power, of determining if the vehicle speed exceeds the set brake cruise speed when the at least one auxiliary brake is delivering full brake power, and of applying a service brake of the vehicle to reduce the speed of the vehicle to a predefined first speed if the vehicle speed exceeds the set brake speed when the at least one auxiliary brake is delivering full brake power, are disclosed.
With the inventive method, an optimized behaviour of the cruise control system of a vehicle can be obtained, in which an undesired acceleration of the vehicle can be avoided during downhill travel when the auxiliary brake cannot produce sufficient brake power. In this way, the fuel efficiency of the vehicle can be increased and the safety can be increased.